THE DEVELOPMENT OF THE PRINCIPAL ARTERIAL STEMS IN THE 



FORELIMB OF THE PIG. 1 



INTRODUCTION. 



The study of the development of blood-vessels has issued in attempts to formu- 

 late the underlying principles that govern such development. Curious to record, 

 the principles so formulated have traveled in a circle; the recent ideas expressed 

 by Evans (1911) are in rough agreement with those expressed by Baader in 1866. 

 The history may be briefly told. 



Baader believed that arterial anomalies were not mere accidents and that the 

 explanation of their occurrence was to be found in the net-formation which precedes 

 the establishment of arteries and veins. Vascular anomalies occur when some 

 part of the net, which normally does not do so, happens to be transformed into a 

 more adult arrangement. Baader arrived at the idea of a capillary net preceding all 

 vessels from the diversity in the anatomical relationships presented by arterial 

 variations. The same hypothesis was upheld by Aeby (1871) and Krause (1876) 

 and it is often referred to as the Baader-Krause law. The weak point in the doc- 

 trine was the absence of direct evidence and when later embryological investigation 

 seemed to point in another direction it failed any longer to command support. 



As soon as it was realized that embryology did not substantiate the idea of 

 a vascular net out of which vascular stems develop in a more or less fortuitous 

 manner, but revealed the presence of only a single main axial trunk, the comparative 

 anatomists imposed a new interpretation on the vascular pattern, in which phylo- 

 genetic and ontogenetic factors were the determining agencies. This took two 

 forms. Macalister (1886) and Mackay (1889) interpreted this main axial trunk 

 as the fusion of an original poly segmental supply to the limb. This idea also found- 

 ered because it was unsupported by any direct evidence. Ruge and others, on the 

 other hand, regarded each arterial stem as a unit and brought direct evidence to show 

 that the axial supply to the limb was such a unit. Ruge (1883), as the result of his 

 study of a 25-mm. human embryo, opposed the idea that blood-vessels arise from 

 a primordial vascular net. "It can be proved," he says, "that the blood-vessels 

 of the upper extremity, as well as for all parts of the body, show themselves 

 differentiated into definite paths in the same manner as the paired aorta?. That 

 at no time does a chaotic mix-up govern the vascular system." In this interpre- 

 tation Ruge was followed by Hochstetter (1890a) and others, and so was elab- 

 orated the doctrine of arteries that regarded each stem as of unit value to be inter- 

 preted in terms of phylogeny. Zuckerkandl (1894, 1895) showed that the volar 

 interosseous artery of the forearm is phylogenetically the oldest artery. In Orni- 

 thorynchus it forms the direct continuation of the brachial. In marsupials, eden- 

 tates, carnivores, and ungulates the arteria mediana appears as the largest vessel, 



1 The author was enabled to carry out this work in America through a fellowship generously granted by the Rockefeller 

 Foundation. 



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